1. Field of the Invention
The present invention relates to a dynamic pressure, air bearing type polygonal scanner rotatable at high speed in a hermetically closed space and a method of producing the same.
2. Description of the Background Art
Today, a digital copier, laser printer or similar electrophotographic recording apparatus with laser optics is extensively used because of high image quality, high-speed printing and low noise as well as cost reduction. The laser optics includes a polygonal scanner. The prerequisite with the polygonal scanner is that it can rotate at a speed matching with the printing speed and pixel density of the recording apparatus.
More specifically, the polygonal scanner is required to rotate at a speed as high as 20,000 rpm (revolutions per minute) to meet the increasing demand for high-speed printing and high pixel density. Such high-speed rotation is not practical with a traditional ball bearing type polygonal scanner because of limited service life and noise particular thereto. A dynamic pressure, air bearing type polygonal mirror is a substitute for the ball bearing type polygonal scanner and configured to meet the above demand.
Japanese Patent Laid-Open Publication No. 7-190047, for example, proposes a high-speed rotary body for a dynamic pressure, air bearing. The rotary body is configured to maintain a preselected rotation speed from the beginning of rotation and preserve high rotation accuracy up to ambient temperature for use. Specifically, the rotary body includes a stationary ceramic shaft and a ceramic sleeve constituting a dynamic pressure, air bearing together with the shaft. The sleeve has uniform thickness in the radial direction thereof. A metallic, hollow cylindrical member is shrinkage-fitted on the outer periphery of the sleeve and has a greater coefficient of linear expansion than the sleeve. After the cylindrical member has been shrinkage-fitted on the sleeve, the inner periphery of the sleeve is machined in a hand-drum shape.
A centrifugal stress acts on the sleeve in the radial direction due to the rotation speed of the rotary body. In addition, a compression stress acts on the sleeve due to the shrinkage-fitting although it is reduced by thermal expansion due to friction. The hand-drum shape of the sleeve is so determined as to maintain a gap between the stationary shaft and the sleeve uniform in accordance with the above stresses.
The high-speed rotary body described above has the following problems left unsolved. When a polygonal mirror with finished mirror surfaces is press-fitted on the sleeve, the mirror surfaces are distorted due to the compression stress. The distortion degrades the flatness of each mirror surface. Even if the mirror surfaces are machined after the shrinkage-fitting, temperature elevation of the rotary body during high-speed rotation cancels the compression stress because the sleeve has a smaller coefficient of linear expansion than the cylindrical member. As a result, the mirror surfaces are again distorted, bringing about the same problem.
In light of the above, the polygonal mirror may be positioned at a higher level than the top of the sleeve in the axial direction, so that the compression stress ascribable to the shrinkage-fitting does will not be transferred to the mirror surfaces. This, however, makes it impossible to locate the polygonal mirror at a lower level than the top of the sleeve and thereby limits the position of the mirror in the axial direction in the optical layout of laser optics.
Another problem with the high-speed rotary body is that machining oil remains between the sleeve and the cylindrical member. The machining oil flows out during rotation and smears the polygonal mirror, a piece of glass and other optical parts.
The rotary body may have its center of gravity located at the center of the dynamic pressure bearing in the axial direction in order to reduce the unbalanced oscillation of the rotary body. In this case, however, when the polygonal mirror is positioned at a higher level than the top of the sleeve, the center of gravity of the rotary body concentrates on the top of the sleeve.
It is an object of the present invention to provide a dynamic pressure, air bearing type polygonal mirror capable of solving the problems discussed above, and a method of producing the same.
A polygonal scanner of the present invention includes a rotary body including a ceramic rotary sleeve and a metallic cylindrical member shrinkage-fitted on the rotary sleeve. The cylindrical member is formed with mirror surfaces constituting a polygonal mirror. A dynamic pressure, air bearing supports the rotary body. The mirror surfaces overlap the rotary sleeve in the axial direction of the rotary sleeve. A hole greater in diameter than the dynamic pressure, air bearing is formed in the top wall of the cylindrical member. The cylindrical member includes a stress removing portion positioned between the mirror surfaces and the rotary sleeve for removing a stress ascribable to shrinkage-fitting.
A method of producing the above polygonal scanner is also disclosed.